There are devices presently available to detect acoustic signals from the chest each having its own advantages and disadvantages as described in the review by Semmlow and Rahalkar. The motivation for most of these devices is the detection of sound signatures associated with coronary artery disease as originally described by Semmlow el al. in 1983. Coronary artery disease results from occlusions or blockages of the coronary arteries which supply blood to the heart. Such blockages will produce turbulent blood flow including an auditory correlate. Theoretical studies by Wang et al indicate that said auditory correlates will be at relatively high frequencies: above 200 Hz and as high as 1 kHz. Such sounds are generally too faint and at too high a frequency to be heard through a traditional stethoscope, although murmurs associated with coronary artery disease have occasionally been reported. Acoustic detection of the sounds produced by blood flowing through partially occluded coronary arteries would thereby enable the noninvasive detection of this major disease.
Devices for the detection of cardiac sounds from the chest fall into two broad categories: those that reference the acoustic energy to fixed positions on the chest and those that use an inertial reference; i.e., accelerometers. Most of the existing devices to measure sounds from the chest are chest-referenced such as described in U.S. Pat. Nos. 6,152,879, 6,261,237, and 7,520,860. Some chest-reference microphones have been constructed in arrays of multiple microphones as described in U.S. Pat. Nos. 6,278,890 and 7,037,268. It is also possible to combine multiple sensors to improve the signal level as described in U.S. Pat. No. 8,715,206. Chest-referenced devices require a mechanical means for stabilizing the sensor on the chest which places a relatively heavy mechanical load on the chest. Modified chest-referenced microphones have been constructed using flexible piezoelectric sensors which are attached directly to the chest using some type of temporary adhesive as described in U.S. Pat. Nos. 5,807,268, 5,595,188, 5,827,198, 5,885,222 and 5,913,829. In this approach, differential movements of the chest under the flexible piezoelectric sensor act as a self-reference.
All such chest-referenced devices must of necessity place a moderate-to-heavy load on the chest. Moreover, traditional microphone designs are sensitive to ambient noise from the environment. Flexible adhesive sensors are less sensitive to ambient noise and induce the lightest load, but they do not detect compression waves and are less sensitive to shear waves. Moreover, in addition to the weight of these sensors, there is still considerable mechanical loading as the chest must force flexion in these sensors in order to detect the bioacoustic energy. Mechanical loading decreases the sensitivity of the detection apparatus particularly to high-frequency acoustic signals as documented by Vermarien and Vollenhoven. To improve the sensitivity of chest referenced microphones, efforts to match the acoustic impedance of the microphone to the chest have been attempted as described in U.S. Pat. Nos. 6,152,879 and 6,278,890. While such impedance matching techniques may improve the power transferred to the microphone, they actually reduce signal level and still place a load on the chest.
Studies using chest reference microphones have not shown the ability to consistently detect the acoustic signatures associated with coronary artery disease as summarized in Semmlow and Rahalkar. An accelerometer-based sensor described by Padmanahban et al. and in U.S. Pat. Nos. 5,036,857 and 5,109,863 has produced signals that were moderately successful in detecting coronary artery disease as shown by Akay et al. Other accelerometer-based sensors have been described in U.S. Pat. Nos. 7,998,091, 8,024,974, and 8,333,718. Although these detectors will present a reduced mechanical load on the chest compared to chest-referenced devices, they are still comparatively heavy. Even moderate loads of 10-15 gm produced by these devices will reduce their ability to detect the acoustic signature of coronary artery disease.
Since mechanical loading the chest will reduce the bio-acoustic signal, particularly at high frequencies, there is a clear need for an acoustic detector which presents a very light mechanical load to the chest. Specifically, the detector should be less than 10 gm so as to be sensitive to the relatively high frequency signals in the range of 200 to 1200 Hz. Said detector should also be relatively immune to ambient or environmental noise.
The quality of the signal produced by any cardiac microphone will also depend on microphone position and attachment to the chest along with patient factors such as body weight. Hence the quality of signals produced by any detector will vary from patient-to-patient and even measurement-to-measurement. The signals produced by all cardiac microphones developed thus far provide no feedback on the quality of the detected signal. There is therefore a clear need for a detector which provides quantitative information on the quality of the acquired signal over the range of desired frequencies. The present invention accomplishes these objectives.